Camera follow object and then stop in place
Yes, you can't animate that, stop using the constraint and keep its transforms. You can do it by hand, using the apply menu, but that is not animatable, imho.

You could do this differently: create two empties, and animate them in a way that they have the same movement until a point, where one stops and the other goes on until the end. 

Then parent the target object to the last empty, and the camera to the first.

This is how it works:



If needed you could add a tracking constraint on the camera to keep focus on the target object, even after it stops.